WASHINGTON (Reuters) - U.S. Environmental Protection Agency chief Lisa Jackson announced on Thursday that she will be stepping down from her post after a tenure of almost four years marked by battles with Republicans who fought her proposed environmental regulations.

Under her leadership, the agency declared for the first time that carbon dioxide was a pollutant that could be regulated under the Clean Air Act, leading the EPA to develop a new regulatory regime to limit carbon emissions.

President Barack Obama thanked Jackson for her service, praising her work on mercury pollution limits, fighting climate change and helping set new fuel economy standards.

"Under her leadership, the EPA has taken sensible and important steps to protect the air we breathe and the water we drink," President Barack Obama said in a statement.

Jackson's time at the agency was marked by some bruising encounters with Republican lawmakers, who accused her of orchestrating massive regulatory overreach that they said was choking U.S. economic growth.

In her statement Jackson, the first black administrator of the EPA, said she was "confident the (EPA) ship is sailing in the right direction."

Leading the list of potential replacements for Jackson are: Bob Perciasepe, the current deputy EPA administrator; and Kathleen McGinty, a former head of Pennsylvania's Department of Environmental Protection and a protege of former U.S. Vice President Al Gore.

"Lisa Jackson has shown an unwavering commitment to the health of our families and our children," Obama said.

Jackson's shepherding of new fuel economy standards "will save the average American family thousands of dollars at the pump," Obama said.

Reports in recent weeks have suggested that Jackson, a chemical engineer by training, might be considering a position at a major university.